The synchronous optical network (SONET) is a standard for a synchronous telecommunication signal used for optical transmission, based on the synchronous digital hierarchy (SDH). SONET is a physical carrier technology, which can provide a transport service for ATM, SMDS, frame relay, T1, E1, etc. As well, SONET provides the ability to combine and consolidate traffic from different locations through one facility (grooming), and reduces the amount of back-to-back multiplexing. More importantly, network providers can reduce the operation cost of their transmission network by using the notably improved operation, administration, maintenance and provisioning (OAM&P) features of SONET.
The SONET standards ANSI T1.105 and Bellcore GR-253-CORE, define the physical interface, optical line rates known as optical carrier (OC) signals, a frame format, and an OAM&P protocol. The user signals are converted into a standard electrical format called the synchronous transport signal (STS), which is the equivalent of the optical signal. The STS-1 frame consists of 90 columns by 9 rows of bytes, the frame length is 125 microseconds. As such, an STS-1 has a bit rate of 51.840 Mb/s. Higher rates (STS-N, STS-Nc) are built from this one, and lower rates are subsets of this. The lower rate components, known as virtual tributaries (VT), allow SONET to transport rates below DS3.
Requests and acknowledgements for protection actions are transmitted in the APS bytes in the SONET overhead, i.e. K1 and K2 bytes. The K1 byte communicates a request for a switch action. The first four bits of K1 indicate the switch request priority and the last four indicate the destination ring node identification (ID). The K2 byte indicates an acknowledgement of the requested protection switch action. The first four bits of K2 indicate the source ring node ID and the last four bits indicate the action taken by that node.
A SONET add/drop multiplexer multiplexes various STS formatted input streams onto optical fiber channels. The STS signals are carried by an optical carrier, which is defined according to the STS that it carries. Thus, an STS-192 signal is carried by an OC-192 optical signal.
The topology of the SONET network can be a linear point-to-point configuration, or a ring configuration.
A linear topology can only protect against single fiber link failures. A “1:1” linear system has an equal number of working and protection links; a “1:N” linear system has N working channels and one shared protection channel.
Lately, rings have become the topology of choice in fiber deployment. The prime motivator for rings versus linear transport is higher survivability. A ring protects against simultaneous failure of the protection and working fibers (i.e. cable cuts) and saves intra-ring and inter-ring pass-through traffic during node failure/isolation. Rings offer cost effective transport while delivering enhanced network survivability.
Currently, two type's of rings are used, namely, unidirectional path switched rings (UPSR), and bidirectional line switched rings (BLSR). The UPSRs are currently used in access networks and therefore they are built for lower rates, such as OC-3, which are sufficient for access link demands. UPSR protection switching is done at the SONET path level. The operation of UPSRs is standardized by the BellCore GR-1400-CORE standard, and there are OC-3/12 rate products available.
The BLSR are currently used in the backbone networks and therefore they are built for higher rates such as OC-48. Switching is done at the SONET line layer. The operation of BLSRs is standardized by the BellCore GR-1230-CORE standard, and there are OC-12/48 rate products available.
The asynchronous transfer mode (ATM) forms the basis for switching in broadband networks. ATM convergence functions permit switching of voice, video and data traffic through the same switching fabric. It multiplexes user information into fixed lengths cells of 53 bytes, with 5 bytes forming a header.
With the constant growing needs for enhanced services in information transmission networks, more efficient transport for bursty traffic carried in ATM cells is needed. There is also a need to simplify and standardize the access link while also obtaining protection of the access traffic. Current practice is to dedicate an entire facility to these new services, such as one STS-1 per customer, where the entire payload is cell based.
While synchronous transfer mode (SIN) access traffic can be advantageously carried on a UPSR, there are disadvantages for ATM traffic. To carry ATM traffic and have the transport vehicle benefit from, and not restrict the bandwidth on demand feature that ATM can provide, the optimum approach is to share a large block of bandwidth between nodes around the ring. In this way, a virtual path (VP) added at a node uses the bandwidth it needs within the large block, rather than using, say, a virtual tributary (VT) where its burst rate will be significantly limited. The shared bandwidth block could be an STS-Nc, where N=1, 3, 6, or higher rates.
This assumes a UPSR with an STS-Nc granularity selector at the tail-ends. With an STS-Nc passing around the UPSR from node to node, the ring bandwidth is quickly exhausted, as each node must source the STS-Nc in different timeslots in order to leave protection timeslots available for other nodes. The UPSR could, in theory, reuse the same working timeslots for the STS-Nc between nodes by operating unprotected and leaving protection to the ATM layer. However, no standardized scheme yet exists at the ATM layer which can provide the 60 ms protection speeds typical of SONET. It is apparent that the two goals of bandwidth efficiency and SONET protection are mutually exclusive in the context of ATM traffic on a UPSR.
A BLSR can carry an STS-Nc node to node in a bandwidth efficient manner. As well, the BLSR can protect any service type since it switches at the line layer. However, the BLSR is not an economical solution for ATM access. This is because for an OC-3 line rate ring, a two-fiber BLSR (2F-BLSR) is not realizable and four-fiber BLSRs (4F-BLSR) are currently not available at the OC-3 rate. An OC-3 4F-BLSR, if it becomes available, would carry twice the bandwidth of an OC-3 UPSR, but almost doubles the cost of the fiber and equipment required, and would therefore be uneconomical for the majority of access applications. Similarly, an OC-12 UPSR or 2F-BLSR, which is typically the next step in upgrading an access applications, carries four times the bandwidth of an OC-3 UPSR, but again, at greater than double the OC-3 cost.
Due to the working timeslot reuse capability, a BLSR always provides the optimum use of bandwidth for a given traffic pattern. However, a complex automatic protection switching (APS) protocol is necessary, which results in longer switching times than for a UPSR. In addition, protection is not optional on a per-path basis. It is also to be noted that for a bidirectional homing traffic pattern typical of the access network, a UPSR is as bandwidth efficient as a BLSR. For a mesh traffic pattern typical of the backbone network, a BLSR is more bandwidth efficient.
In addition, regardless of the line rate or 2F/4F type, a BLSR must perform the ATM add/drop functionality for two bidirectional channels (e.g. east and west). ATM chip sets available today are designed for terminals, or a single bidirectional channel (e.g. east or west). It is expected that the evolution to ATM chips consolidating add/drop functionality for two bidirectional channels will be eventually available, but again at higher costs.
In conclusion, there is no standardized survivable access vehicle currently available which can efficiently carry ATM or mixed ATM/STM traffic.